Oil psi and oil temp

Long long ago in a galaxy far far away i worked in a shop fabricating race cars for a local track here in so cal.. there was a old dude who built the engines for the cars that were to be raced.. one day he and I were talking and he said to me that most of the heat in the oil comes from the oil pump via heat of compression and the basic rule for pressure was 10 psi per every 1000 rpm..
Has anyone heard of similar values and theory for aviation engines, if there is merit to them then most aviation ( lycoming engines for this matter) need not more that 30-35 psi at 2700 rpm.. . Does less psi = less heat. And still have safe margin for operation

Heat of compression? That makes sense for a gas, but not a liquid. Plenty of heat coming from the combustion cycle though.

30 psi is considered on the low side for oil pressure, and there is an argument that significantly higher pressure in the 80-90 psi range is beneficial in lycomings to increase lubrication of the camshaft lobes

billytime1 said:

Long long ago in a galaxy far far away i worked in a shop fabricating race cars for a local track here in so cal.. there was a old dude who built the engines for the cars that were to be raced.. one day he and I were talking and he said to me that most of the heat in the oil comes from the oil pump via heat of compression and the basic rule for pressure was 10 psi per every 1000 rpm..
Has anyone heard of similar values and theory for aviation engines, if there is merit to them then most aviation ( lycoming engines for this matter) need not more that 30-35 psi at 2700 rpm.. . Does less psi = less heat. And still have safe margin for operation

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Er, no. There is heat produced from "extreme pressure" because the two gears actually make contact versus the hydrodynamics assoc with bearings, etc. There's other friction as well but relatively not very much.

Heat introduced into the oil vis pumping is directly proportional to the pump's (in)efficiency. External gear pumps are typically in the mid 80's, % efficiency wise. Again, not much wasted energy turning into heat, noise, etc. If you're compressing gas where most of the energy is being lost to heat, the efficiency could be as low single digits if the system doesn't recuperate some of the heat.

Short story made long; one of my superpowers. Your friend appears to be a clever guy and made an observation and a relationship that worked for him; however, the reasoning behind it was off. We've all been guilty of that.

BTW. Stick with the OEMs recommendation for oil pressure. Don't think anyone here will argue with that. Stay safe Sir.

I don't buy that and goes against everything that I was taught about engines.

Engine oil is splashed on the cyl wall AND the piston bottom. The latter portions of oil are pulling a decent amount of heat out of that piston. Pistons get just as hot as the head does and that heat needs to be removed in some way. Remember that there is VERY little metal to metal contact to transfer that heat to the cyl wall, so we need supplemental cooling and that is what the oil does for us. The oil that is splashed on the walls also contacts the piston on the down stroke and pulls some heat as well.

air cooled engines require the oil to remove even more heat, as they cannot remove it is efficiently as a water cooled engine. This is why so many air cooled engines have oil coolers.

I have no doubt that heat is generated in the pump. I do not believe that is where the majority of oil heating comes from though. While I agree that the lyc's use much higher pressures than normal relative to water cooled, I believe that the higher pressures provide a greater flow volume and this is spec'ed because of the oil's requirement to do more heat removal than a water cooled engine. Higher pressure = higher flow volume = greater heat absorption by the oil = greater heat removal from rotating parts.

Larry

Just to put an order of magnitude on things, power put into a fluid is flow rate times pressure rise. If you move 10 gpm at 10 psi, do the unit conversions and that's about 0.06 hp. Move that same flow at 100 psi and it's about 0.6 hp. Engine efficiency varies, but is less than 50%, so if you're running 150 hp to a propeller then you're dumping in excess of 150 hp as waste heat. You dump this out of the engine through multiple paths such as exhaust gas and oil temperature rise across the engine but ultimately all of this energy can only leave the engine compartment via heat transfer off the cowling (convection and radiation) and air flow through the compartment (air temperature rise, mass flow rate). That's glossing over a lot of details, but oil pump energy is a relative fart-in-the-wind in the overall energy/thermal situation.

If you *really* care, you can measure oil temperature before and after the pump, and before and after the cooler. This would tell you the relative contribution of the pump and engine to oil temperature rise. I wager you'll find the temperature change over the pump almost lost in the accuracy of measurement.

Wow, this gives me a head ache!
The oil pump is such a small contributor to the heat picture in our air cooled engines, I believe it is almost unmeasurable.
I like oil pressure in my engine.
And here I thought burning fuel created heat.
Art